Gain switched laser diodes have important applications for marking, cutting, scribing, and other industrial processing of materials that require pulse widths in the picosecond to low nanosecond range. The pulse is generated by modulating the gain of the laser via the pump power. If a high pump power is suddenly applied to the laser, laser emission commences after a short delay. The pulse commences with weak fluorescence light, which is then amplified in a number of resonator round trips. Energy stored in the gain medium is thus extracted in the form of a short pulse. The laser diode can be operated with short electric current pulses, or with a continuously modulated electrical signal. Typical pulse lengths that are achievable range from a few nanoseconds down to a few tens of picoseconds. The pulse repetition rate is controlled electronically, and can be varied over a wide range. It is usual for the laser to be biased below the laser threshold prior to the pump power being increased. A seed laser pulse can be amplified by optical amplifiers to provide pulses having energies in the microJoule or milliJoule range.
USA Patent Application No. 2014/0185643 describes gain switched laser diodes that are used to provide seed pulses for optical pulse generation. Amplified spontaneous emission (ASE) is reduced by applying a prebias to the laser diodes at an amplitude less than that associated with a laser diode threshold. An electrical seed pulse having an amplitude larger than that associated with the laser threshold is applied within about 10-100 ns of the prebias pulse. The resulting laser diode pulse can be amplified in a pumped, rare earth doped optical fiber, with reduced ASE. The patent application describes how the prebias is combined with the seed pulse in a microwave power splitter that includes resistors that are selected to provide suitable split ratios or impedance matching. Unfortunately such impedance matching causes attenuation of the signals that are applied to the laser diode, which in turn affects the pulse energy of the laser emission. This deficiency can be cured using ultra-wide-bandwidth electronic amplifiers to amplify the electrical pulse signal. However, ultra-wide-bandwidth electronic amplifiers are expensive.
There is a need for apparatus and a method for providing optical radiation that avoids or reduces the aforementioned problems.